When measuring ECG, EEG and EMG a plurality of electrodes, each of which have a sensor, are attached to the skin of an animal or human patient. Each sensor on each electrode senses the electrical potential at a relatively small area—typically a few mm in diameter—on the skin immediately below the sensor. Usually an electrically conductive contact medium is used between the sensor and the skin. The sensed electrical potentials are transmitted through individual conductors to an apparatus, which measures, registers and/or evaluates the sensed potentials, e.g. by determining differences between electrical potentials at the electrodes and linear combinations thereof. Such systems have long been used for monitoring and diagnostic purposes.
U.S. Pat. No. 4,583,549 and others disclose an ECG electrode pad comprising a flexible sheet with a plurality of ECG electrodes positioned thereon to correspond with the anatomically correct placement for precordial ECG electrodes.
U.S. Pat. No. 5,724,984 discloses an electrode with a central sensor segment and several peripheral sensor segments arranged symmetrically around the central sensor segment.
U.S. Pat. No. 6,577,893 discloses an integrated wireless medical diagnosis and monitoring equipment with two or more electrodes and a wireless transmitter for transmitting sensed electrode signals.
US 2002/0045836 A1 discloses a method of calculating the standard ECG leads based on a number of smaller measurements. The method is based on arranging a set of sensors in a rectangle with the longitudinal axis along the standard ECG lead. The standard ECG lead measurement is calculated based on a number of assumptions.
U.S. Pat. No. 6,295,466 B1 discloses an electrode comprising three sensors arranged in orthogonal relationship. The output from the three sensors is combined in order to find a vector magnitude describing the magnitude of the change in the electric field at the sensors location. However, no information is provided with respect to the standard ECG or to the direction of the change of the field.
ECG signals originate from the coordinated activation and contraction of the heart muscles resulting in blood circulation through the body. The ECG signal starts at the SA node and initiates the contraction of the atrial myocardium resulting in the P wave, which travels down the centre line of the heart. The AV node and the bundle of His are activated, whereby the activation of the ventricles is initiated. First the septum (the muscle that separates the two ventricles) is activated, which results in the Q wave as the signal is travelling down the centre line of the heart. Then the free outer walls of the ventricles are activated, which results in the R and S waves. There the activation travels from the apex of the heart up the centre line, and completes the QRS complex. The T wave originates from the re-polarisation of the outer walls of the ventricles, actually travelling from the apex upright the centre line, but with inverted signal polarity, resulting in what appears to be a downward movement towards the apex. Finally, a small U wave can be found which originates from a late activation of the ventricles. The shape of the recorded ECG signal depends on the location of the electrodes and the polarity of the recording.
The ECG signal used in diagnostics originates from measuring the difference in the biopotential between different sites on the body. There are several different standards for recording/forming the ECG signal and these will not be discussed here. The reader is referred to any basic medical textbook. In the following, the three standard leads or limb leads (I, II, III) forming the Einthoven's triangle [10] are taken as an illustrative example. Traditionally, the electrode placements of the limb leads are on the right arm (RA), left arm (LA) and left leg (LL). In several clinical applications the limb electrodes are placed on the torso near the extremities without loss of information [5], [8].
The limb leads are defined as potential differences recorded between the three electrodes. Specifically they are:I=VRA−VLA II=VRA−VLL III=VLA−VLL 
where VXX is the potential recorded under electrode XX (eq. I).
Besides the primary standard limb leads, the unipolar limb leads, also called augmented leads, can be calculated from the same three potential recordings as the standard limb leads. Together, the standard limb leads and the unipolar limb lead form a vector system constructed of 6 vectors.aVR=VRA−(VLA+VLL)/2aVL=VLA−(VRA+VLL)/2aVF=VLL−(VRA+VLA)/2
In traditional ECG recordings, ECG signals are recorded as the difference between two potentials at two different sites on the body. The voltage difference is measured relative to a reference point, which is taken as a “zero potential” on the body. This means that the signal is always in relation to a single common point on the body. This reference point can be a single site/electrode placed on a site of the body that is minimally influenced by the body potential of interest, or the reference can be one or several potentials/electrodes. This dependency on a reference point(s) limits the possibilities of transmitting the signal of a single electrode over a non-reference transmission line, e.g. wirelessly. Normally this would only be possible if a relation between at least two electrodes can be made, e.g. with a pair of wires connecting two electrodes to a transmitter.
Traditionally, each sensor has its own conductor that connects the sensor to the measuring and/or evaluating apparatus. With several electrodes and a corresponding number of individual conductors there is a risk of confusing the conductors and of connecting sensors to wrong inputs of the apparatus. A patient with a set of such electrodes attached has his/her mobility restricted by the length of the conductors. In equipment powered by AC mains power supply the electrodes must be extremely well isolated from the AC mains power supply in order to ensure patient safety.
In most currently available systems for telemetric monitoring of patients a set of electrodes are attached to the patient, where each electrode is connected via a conductor to a common transmitter carried by the patient. Such systems also have conductors that restrict the freedom and the mobility of the patient.
There is therefore a need for a disposable electrode, a method of measuring potential differences between different points on the body without the need for a common voltage between the points, and a system that allows wireless transmission of signals representing sensed potentials from each electrode. There is also a need for more detailed information on the electrical biopotential underneath the electrode, such as the direction of propagation of the biopotential.